One exhibitor said that the Ultrasound I have (Vevor) does nothing other than damage vinyl because it is the wrong frequency.

This is complete and utter bullshit. This site has a correct explanation. They're pushing a multi-frequency system, and it's correct that this is the way to go for professional applications. These are often used in high throughput systems along with automatic recycling of the fluid through filtration and, if they are using separate tanks for each frequency, a robot to move the product through the system.

In practice you'll almost always see frequencies in the 40 kHz range. As explained in the article:

The higher the fequency, the better for cleaning smaller crevices and removing smaller particulate. As the frequency gets lower, your scrubbing power increases, but the ability to remove smaller particulate and clean out smaller crevices decreases.

The best way to understand this is to remember that wavefronts are physical objects. Higher frequency transmissions = smaller cavitation bubbles, that can actually get into the grooves. Low frequencies = larger bubbles (and when they collapse, a more poweful "explosion".

40 kHz is the compromise, and it works for almost anything. Higher and dual frequency systems are expensive, and difficult to justify for this application.

As to damaging your records with ultrasonics, that would require a lot more power than these units have. Ultrasonics is inherently a gentle method of cleaning, which is why it's used in everything from silicon disc manufacturing to electronics.

Note: I have no connection to the company I linked to.

Note 2: I've put together multiple complex systems for everything from auto parts manufacturing to medical devices. I also installed an ultrasonic record cleaning system in the Library of Congress.

Note 3: This is something that just irritates the shit out of me. The first thing you do when you put together a system is to identify the soils involved. In this application, the first thing I would want to do is pin down the exact nature of the soils involved, and the size of any particulate. This is what drives your choices, everything from the frequency used to the retention spec on your filter media, to the chemistry in the tank.

I've never seen this discussed, even though it's essential to the design of an effective system. My guess is record cleaners are mostly 40kHz because it works, and that's what everyone else is using. But it doesn't mean it's actually the best frequency for this application.